The present invention relates to compact duplex bearing mounts, particularly for those bearings which must operate with high precision in high vibration environments.
Duplex bearings are well known in the art and may be generally described as having concentric annular inner and outer races which rotate relative to each other, and an annular arrangement of load-supporting rotating elements, such as balls or rollers, located between the races. High precision applications for duplex bearings are often subjected to vibratory conditions which are severe. For example, duplex bearings must operate with high precision during, and after, the launching of a spacecraft, the vibration caused by an airplane propeller, or the operation of a vehicle on a rough road. The vibration loading on the bearing or its mounts can also adversely affect the bearing running characteristics, resulting in jitter. Further, these loads can also affect the individual components comprising the system supported by the bearing, resulting in deformations that change the modal properties or natural frequencies of the system. These changes in natural frequencies, which are indicative of a change in the loading on the bearing, can result in the generation of objectionable, audible sounds by the bearing-supported system, or vibrations that can adversely affect the life of the bearing, or other systems near the bearing.
For high precision rotation applications, such as those requiring very low rotational jitter (on the order of 10 microseconds of rotational period variation per revolution), the preload force applied to the bearing is critical. Variation in preload may result from yielding of the bearing retaining system and will affect the running torque characteristics of the bearing, which can increase jitter and adversely affect the natural frequencies of the system supported by the bearing.
A previous duplex ball bearing mount for a rotary device is shown in the rotary device depicted in FIG. 1A, which is an electric motor assembly. This mount is very susceptible to variations in the tolerances of the individual parts and tolerances in the preload applied through torquing of the bolts. These variations resulted in variations in jitter and natural frequency as the motor was subjected to vibration and changes in its thermal environment.
Motor assembly 30 includes first and second housing portions 32 and 34, respectively, located on and abutting the opposite axial ends of stator 36. Bolts 37 secure the housing portions and the stator together. Rotor 38 is disposed within stator 36, and has an axis of rotation which is colinear with bore 40 extending through the rotor. Shaft 42 is interference fitted within bore 40, rotatably fixing it to the rotor. End 44 of the shaft extends through first housing portion 32, and is operatively coupled to a device (not shown) driven by the motor. Referring to FIG. 1B, shaft 40 is provided with shoulder 46 between portion 48, which is fitted into the rotor bore, and portion 50, which is supported by duplex bearing 52. Referring again to FIG. 1A, at the end of shaft 40 which is opposite end 44, portion 54 is supported by duplex bearing 56, which is identical to duplex bearing 52. Shoulder 58 is located between shaft portions 48 and 54. Shoulders 46 and 58 respectively abut inner race 60 of bearings 52 and 56, through which shaft portions 50 and 54 extend. Outer race 62 of bearing 56 is received in counterbore 64 of second housing portion 34, and abuts annular face 66 thereof, which forms a bearing retainer.
Outer race 62. of bearing 52 is received in counterbore 68 of first housing portion 32. Disposed between annular face 70 of counterbore 68 and outer race 62 of bearing 52 is wave spring 72, shown in greater detail in FIG. 1C. Spring 72 has free height H1 and variable, loaded height H2. As bolts 37 are tightened, spring 72 is compressed to its loaded height H2 and urges shaft 42 leftward as viewed, through bearing 52 and shoulder 46, and clamps the bearings axially. This bearing mount device is rather sensitive to the amount of torque exerted on bolts 37, therefore bolts 37 must be precisely torqued. The bearing mount device, through deflection of spring 72, preloads the duplex bearing axially, in the direction of the shaft axis of rotation. As the bearing is clamped, spring 72 and, to a lesser extent, the other components of the retaining system, will deflect.
Those skilled in the art will recognize that in the type of duplex ball bearing depicted, an axially directed force is transferred between inner and outer races 60, 62 though interfacing, radially offset shoulders located between the inner and outer races. Balls 74, which roll on these shoulders, support both the axial and radial loads exerted on the bearings. Each race of a bearing may be provided with only one annular shoulder, as shown in FIGS. 1A and 1B. Alternatively, one of the inner or outer bearing races may be formed with a circumferential groove forming a pair of shoulders; during assembly of the bearing, the balls are first disposed in the circumferential groove provided in the inner or outer race, and the other race, having a single shoulder, is then positioned so as to capture the balls between the two races. Notably, each duplex bearing must be oriented and assembled into its mount such that axial forces exerted on one of its inner and outer races will be directed through its shoulder and the balls, to the shoulder of the other bearing race; otherwise the bearing may come apart.
As the bearing is clamped, spring 72 will deflect, applying a load to outer race 62 of bearing 52 that is proportional to the deflection. Thus, changes in the deflection of the spring will result in changes in the load on the bearing. FIG. 2 shows the substantially linear relationship between spring deflection and bearing load. The clamping torque, which can be measured as the torque to rotate the inner bearing races relative to their associated outer bearing races, is shown on the abscissa of FIG. 2 and is analogous to the deflection of the spring. Thus, increased torque results in an increased load on the bearing.
The Smalley Steel Ring Company produces wave springs of the type used in motor assembly 30; disc springs and finger disc springs may also used. These spring designs, however, have a load variation on the order of +/xe2x88x9225%. This is not acceptable for high precision mechanisms, which require stable performance, especially over a long period of time under severe environment conditions, as required of space flight mechanisms such as, for example, an Advanced Very High Resolution Radiometer (AVHRR) (not shown), the primary imaging instrument on certain polar orbiting meteorological satellites. In such applications, an open loop scanner is positioned by a motor having a shaft supported by duplex bearings.
FIGS. 4-7 show a number of other previous spring-based concepts by which various configurations of shaft 42xe2x80x2 is supported by a plurality of duplex ball bearings 52, 52xe2x80x2, 56, and/or 56xe2x80x2. Each of these concepts employ coil springs 76, which are more linear in their force versus deflection characteristics vis-a-vis wave spring or disc springs. Coil springs, however, require much more axial space. If coil springs 76 were eliminated from the designs of FIGS. 3-7, however, it would be impossible to clamp the bearings without changing the effective preload. Further, if coil springs 76 were not present, the load applied when inner and outer bearing retainers 78, 80 are clamped would result in deflection of other parts of the system, such as bearing races 60, 62, any bearing spacers 82, or the bearing retainers themselves.
Notably, too, if the resulting stress from the applied clamping load were greater in outer race 62 than in inner race 60, or vice versa, the resulting strain would cause a change in the effective preload of the bearing, which is undesirable.
Although previous bearing mounts employing springs 72, 76 are used to provide a defined point of deflection in a bearing mount, they are not effective for high-precision applications which are required to undergo severe vibrations, which impart high loads to the bearings. For example, during vibration, the bearings of the above-mentioned AVHRR scanner experiences loads approaching 1000 pounds, well beyond the load capacity of existing wave, disk and coil springs given the small amount of axial space available.
Further, unless painstaking measures are taken to control tolerance stackup in the rotary device assembly, previous bearing mounts are largely susceptible to variations in bearing loads. Moreover, unless materials of common coefficient of thermal expansion are used for those parts which comprise the bearing mount system, changes in bearing preload can occur in cases where previous bearing mounts are subjected to changes in their thermal environments.
A high precision duplex bearing mount which overcomes the above-mentioned variability problems, requires less packaging space than previous mounts, is substantially impervious to changes in temperature and tolerance stackup in the rotary device assembly, and is suitable for use in high-vibration environments is highly desirable. Such a bearing mount would be particularly useful wherever high precision bearings are desired, such as, for example, in machine tool and semiconductor manufacturing machinery applications, as well as in aerospace applications.
The present invention applies a precise, controlled clamping load on duplex bearings while providing a mount which can survive high vibration loads. The sensitivity to the torque applied in axially clamping the bearing is also eliminated, providing a more robust bearing mount design vis-a-vis previous bearing mounts. The inventive duplex bearing mount also provides a very compact package requiring little space, and is lightweight, making it particularly useful for use in spacecraft and other applications where required package space and weight must be minimized.
The present invention provides a duplex bearing mount including at least one duplex bearing having an inner race and an outer race, the inner race disposed within the outer race and being rotatable relative to the outer race about an axis, the inner race having substantially no relative movement relative to the outer race in at least one direction along the axis, the inner and outer races each having first and second axial faces which are respectively located at the same axial end of the duplex bearing. The duplex bearing is radially supported by a housing, and a shaft extends through the inner race, the shaft radially and axially supported by the inner race. A first retainer is connected to the housing and engages the first axial surface of a bearing race, the movement of which race in a first direction along the axis being constrained by the first retainer. A second, resilient retainer is connected to the housing or the shaft and is deflected through engagement with the second axial face of a bearing race, the movement of which race in a second direction along the axis, opposite to the first direction, being constrained by the deflected second retainer. The bearing is preloaded by its being clamped between the first and second retainers, and the second retainer forms at least a portion of a spring having the characteristic of a substantially constant force value correlating to a range of various deflection values, whereby the preload of the bearing is substantially unaffected by variations in the deflection of the second retainer.
The present invention also provides a duplex bearing mount including first and second duplex bearings each having an annular inner race and an annular outer race, the inner races respectively disposed within the outer races and being rotatable relative to the outer races about an axis. Relative movement between the inner and outer races of each respective bearing in at least one direction along the axis is constrained, radial movement of each bearing relative to the axis is constrained, and the first and second bearings have fixed relative positions along the axis. A bearing retainer is in operative engagement with the first bearing outer race, movement of the first bearing along the axis in the direction of the retainer being limited by the bearing retainer. An annular member is connected to the bearing retainer and includes a plurality of radially inwardly extending resilient tabs which engage the second bearing outer race and are deflected thereby, the first and second bearings being urged in a direction along the axis toward the bearing retainer. The annular member at least partially defining a nonlinear spring having the characteristic of a substantially constant force value corresponding to a range of different deflection values, whereby a constant preload can be exerted on the duplex bearings under varying amounts of tab deflection.
The present invention also provides a duplex bearing mount for a rotary device, including a pair of duplex bearings each having an inner race and an outer race, the inner race disposed within the outer race and being rotatable relative thereto about an axis, the inner races and the outer races respectively engaged with each other along the axis. A bearing retainer is engaged with the outer race of one of the duplex bearings, movement of the duplex bearings in one direction along the axis being limited by the bearing retainer. Also included are means for providing a substantially constant preload on the duplex bearings under varying amounts of displacement of the duplex bearings in directions along the axis.